linux-kernel - Re: [PATCH 22/25] Generic dynamic per cpu refcounting

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Message-ID: <20121129205920.GL15094@google.com>
Date:	Thu, 29 Nov 2012 12:59:20 -0800
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Benjamin LaHaise <bcrl@...ck.org>, linux-kernel@...r.kernel.org,
	linux-aio@...ck.org, linux-fsdevel@...r.kernel.org, zab@...hat.com,
	jmoyer@...hat.com, axboe@...nel.dk, viro@...iv.linux.org.uk
Subject: Re: [PATCH 22/25] Generic dynamic per cpu refcounting

On Thu, Nov 29, 2012 at 09:54:47PM +0100, Andi Kleen wrote:
> > > The regular atomic_t is limited in ways that you are not.
> > > See my original mail.
> > 
> > I don't follow, can you explain?
> 
> For most cases the reference count is tied to some object, which are
> naturally limited by memory size or other physical resources.
> 
> But in the assymetric CPU case with your ref count no such limiter
> exists.

It's got exactly the same limit as the old code which used the atomic_t
- we're limited by the number of threads that can be issuing aio
syscalls at a time.

The assymetry you're talking about _doesn't matter_, individual cpu
counters wrapping does not affect what the counters all sum to when we
go to tear down.

A coworker at lunch actually pointed out to me that the reason this is
true is just that modular arithmatic is still associative with addition
and subtraction.
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